The content of H+ or H3O+ ions in a measured medium, especially the activity and/or concentration of these ions in the measured medium, is an important measured variable in environmental analytics as well as in a number of chemical or biochemical processes in the laboratory or in industrial processes. The H+ ion content of a measured medium, as a rule, is given as a dimensionless pH value, which is formed by the negative base 10 logarithm of the H+ ion activity in the measured medium. To a first approximation, in dilute solutions, the H+ ion activity can be set to equal the H+ ion concentration. Analogously to the pH value, the pOH value is defined as the negative base 10 logarithm of the OH− ion activity, or, to a good approximation, of the OH− ion concentration in dilute solutions. The two values are related to one other through the constant ionic product of waterpH+pOH=14.
From the pH value or the pOH value, thus, the associated H+ ion or OH− ion activities and/or the corresponding concentrations can be correspondingly ascertained.
The most popular technology for determining the pH value of a measured solution is potentiometric measurement. A single rod, measuring chain frequently referred to as a glass electrode serves most frequently as a measuring transducer for the potentiometric measurement of the pH value; the single rod, measuring chain combines a pH sensitive, measuring half cell and a reference half cell, which provides a stable reference potential, which is independent of the pH value of the measured medium. The measuring half cell comprises, as a rule, a tubular glass housing closed on one end by a membrane comprising a pH sensitive glass; the tubular glass housing is filled with an internal electrolyte, for example, a buffer solution containing chloride; a potential sensing element, for example, a chloridized silver wire, extends into the buffer solution. In contact with the measured medium, a measuring half cell potential dependent on the pH value forms at the glass membrane. Serving, as a rule, as reference half cell is a reference electrode of second type, for example, an Ag/AgCl or calomel electrode. The potential difference between the measuring half cell potential tappable at the potential sensing element of the measuring half cell and the reference potential (which is ideally independent of the pH value of the measured medium) of the reference half cell forms the primary signal of the measuring transducer and is a direct measure for the pH value of the measured medium.
Although such potentiometric measuring transducers assure very precise and reliable measurement results and are well established both in the laboratory as well as in process analytics, they have a number of disadvantages. Especially the very thin, pH sensitive, glass membrane is very difficult to manufacture and is extraordinarily sensitive to handling.
Breaking the glass membrane can lead to glass shards getting into the measured medium. If the measured medium is, for example, a product, or an intermediate product, manufactured in a pharmaceutical or food process, the measured medium must be discarded upon such a glass fracture, in order to avoid endangering the end consumer by glass shards in the product.
Due to the small conductivity of the pH sensitive glass membrane, it is additionally necessary to measure the potential difference between the leads of the measuring transducer with very high impedance, which can lead to instabilities in the measuring and corruptions of the measured value. Due to the high resistance of the glass that forms the glass membrane, miniaturization of the pH glass electrode is limited, since the resistance of the measuring half cell continually rises with the reduction of the area of the glass membrane. There has long been the need for alternative measuring methods using more robust measuring transducers for determining the pH or pOH value.
Recently, above all, measuring of the pH value by means of an ion selective, field effect transistor (ISFET) and optical pH measurement have achieved a certain importance.
An ISFET measuring transducer includes an ISFET semiconductor chip, which basically corresponds to a MOSFET in its construction, wherein the metal gate, in the case of the ISFET, is replaced by an ion selective layer, for example, an H+ ion selective layer. Such an ISFET has a semiconducting substrate, for example, an n-conducting substrate, in which two p-conducting areas, which function as the source and the drain of the ISFET, are embedded. The area between the source and the drain serves as a semiconductor channel, which is isolated from the ion selective layer by an insulation layer. If the ion selective layer is impacted by the measured medium, its pH value influences the number of charge carriers in the semiconductor channel between the source connection and the drain connection. ISFETs can thus be considered electronic components, whose threshold voltage can be changed by the surface potential formed at the oxide, measured liquid interface by the contacting of the pH sensitive coating with the measured medium. By exploiting this effect, a measurement circuit of an ISFET measuring transducer produces an electrical output signal, which is correlated with the pH value of the measured liquid. A pH measuring device having a measuring transducer comprising an ISFET is described, for example, in DE 19857953 C2.
The mechanical stability of an ISFET measuring transducer is, indeed, greater than that of a glass electrode; however, the known pH sensitive, metal oxide layers are sensitive to aggressive measured media, for example, strongly alkaline measured media, especially at increased temperatures. Additionally, ISFET measuring transducers cannot be sterilized by irradiating with beta or gamma radiation, as is common, for example, in medicinal or biotechnological applications.
Measuring transducers for optical pH measurements comprise a pH indicator immobilized in a sensor matrix; the pH indicator's optical properties, for example, its absorption spectrum or fluorescence spectrum, are influenced by the pH value of the measured medium contacting the indicator. Frequently, a polymer membrane or a porous solid structure serves as the sensor matrix. Measuring transducers for optical pH measuring further comprise an optical measuring arrangement for registering the influenced optical property. The optical measuring arrangement is suitably embodied, depending on the type of indicator and the optical property influenced by the pH value. If the influenced optical property is, for example, the intensity of a fluorescent emission by the indicator at a fixed wavelength, the optical measuring arrangement can have, for example, one or more radiation sources for initiating the fluorescence, and one or more photoelectric elements for registering the fluorescent radiation at, in given cases, a fluorescence wavelength. The photocurrent output by the photoelectric element or an electrical signal derived therefrom forms the measurement signal of the optical pH measuring transducer. Such measuring transducers are described, for example, in DE 10 2008 033214 A1. Optical measuring transducers for pH measurement are robust and relatively inexpensive to manufacture. However, many additional factors, especially cross sensitivities to other ions present in the measured medium, influence the immobilized indicator and so corrupt the measurement result.